1. Field of the Invention
The present invention relates to an exhaust emission control apparatus and method for an internal combustion engine.
2. Description of the Related Art
There have hitherto been known a technique in which when the amount of sulfur deposited on an NOx catalyst exceeds a predetermined amount, a nasty or unpleasant smell issued by the deposited sulfur is suppressed by varying the air fuel ratio of a mixture in an increasing or decreasing sense around a reference rich air fuel ratio (for example, see a first patent document: Japanese patent laid-open No. 2000-274232 (pages 3-5 and FIGS. 2 and 3)), another technique in which recovery of an NOx catalyst from its SOx poisoning is carried out at the time of idling or deceleration of a vehicle (for example, see a second patent document: Japanese patent laid-open No. 2002-38932 (pages 4-8 and FIGS. 3 and 4)), a further technique in which the air fuel ratio of a mixture is changed between a rich ratio and a lean ratio so as to recover an NOx catalyst from its SOx poisoning when the discharge or release speed of hydrogen sulfide in the course of recovery of the NOx catalyst from its SOx poisoning exceeds a predetermined speed (for example, see a third patent document: Japanese patent laid-open No. 2001-81237 (pages. 3-7 and FIGS. 2, 3 and 4)), and a still further technique in which the recovery of an NOx catalyst from its SOx poisoning is not performed at the time of idling or deceleration of a vehicle (for example, see a fourth patent document: Japanese patent laid-open No. 2000-161045 (pages 3-9 and FIGS. 5, 6 and 7)).
However, when an NOx catalyst is recovered from its SOx poisoning during the idling operation of an internal combustion engine, it will take time for the hydrogen sulfide discharged into an ambient atmosphere to be sufficiently diluted by or dispersed into the atmosphere, and hence there might be generated a nasty or unpleasant smell due to the hydrogen sulfide discharged into the atmosphere.